1. Field of the Invention
The present invention relates to a method of making a semiconductor film having a uniform thickness and a uniform impurity concentration, and an apparatus for making the same.
2. Description of the Related Art
For providing semiconductor devices, semiconductor films have been often formed on substrates, and a semiconductor film of polysilicon has been particularly widely utilized as electrodes or wiring layers. The semiconductor film has been provided by the LPCVD (Low Pressure Chemical Vapor Deposition) method which is on of the most popular methods.
Conventionally, a semiconductor film of doped polysilicon has been formed as described below. An undoped polysilicon film is first deposited on a substrate using the LPCVD method, and an impurity is then introduced thereinto using thermal diffusion of ion-implantation techniques.
In order to further decrease the number of steps of forming a semiconductor film, the following deposition method has been used. That is, when a polysilicon film is deposited on a substrate using the LPCVD, an impurity is simultaneously added to the semiconductor film of polysilicon.
A conventional method of forming a semiconductor film of polysilicon will be described below with reference to FIGS. 6 and 7.
FIG. 6 is a schematic view showing an apparatus for forming a semiconductor film, and FIG. 7 is a graph showing a relation between an impurity concentration in the semiconductor film and a deposition rate of the semiconductor film. Referring to FIG. 6, the apparatus for forming the semiconductor film will be described. Reference numeral 1 denotes a vertical reaction chamber which places substrates such as a plurality of semiconductor substrates 2 on which a semiconductor film is deposited under a reduced pressure. A heater 3 is arranged around the side wall to heat the inside of the reaction chamber 1 to a predetermined temperature. The upper portion of the reaction chamber 1 is provided with an exhaust port 4 which is connected to a vacuum pump (not shown) for exhausting the inside of the chamber t reduce a pressure to a predetermined value. A first gas supply pipe 5 and a second gas supply pipe 6 for supplying gases to the reaction chamber 1 are arranged at the lower portion of the side wall of the reaction chamber 1. A raw gas such as monosilane (SiH.sub.4) is introduced into the reaction chamber 1 through a first valve 9 from a supply port 7 of the first gas supply pipe 5, while an impurity gas such as phosphine (PH.sub.3) is introduced through a second valve 10 from a supply port 8 of the second gas supply pipe 6 thereinto. Each gas supply source is arranged at one end of the corresponding one of the gas supply pipes. Reference numeral 11 denotes a quartz boat standing in the reaction chamber 1, and a plurality of semiconductor substrates 2 are horizontally held on a plurality of shelves of the boat 11 one by one. At this time, a space for circulating the gases is provided between the substrates 2.
A method of forming a semiconductor film on a substrate by using the apparatus arranged as described above will be described below.
A plurality of semiconductor substrates 2 held on the boat are placed in the reaction chamber 1, and the reaction chamber is evacuated by the vacuum pump to have a predetermined pressure and kept at a predetermined temperature by the heater 3. Subsequently, while the reaction chamber is evacuated by the vacuum pump, the first valve 9 and the second valve 10 are opened, and monosilane and phosphine are simultaneously introduced into the reaction chamber 1 at corresponding predetermined flow rates through first and second supply pipes 5 and 6. The polysilicon film having a predetermined thickness and containing an impurity of phosphorus is deposited on the semiconductor substrate 2. Thereafter, the valves are closed, and the vacuum of the reaction chamber 1 is broken to provide the semiconductor substrates 2 each having the phosphorus-doped polysilicon film.
However, there are provided the following problems when the phosphorus-doped polysilicon film is formed as described above. In FIG. 7, the abscissa represents a distance between the gas supply port to a substrate 2, and the ordinate represents the impurity concentration in a semiconductor film and a deposition rate of the semiconductor film. As is apparent from FIG. 7, the impurity concentration and the deposition rate of the polysilicon film are changed according to positions of the semiconductor substrates 2 in the reaction chamber 1, i.e., the distances from the gas supply ports 7 and 8 at corresponding ends of the first and second gas supply pipes 5 and 6 to the semiconductor substrates 2. That is, the thicknesses of films formed on semiconductor substrates are not uniform due to the following reasons. When the impurity (P) is deposited on the surface of the semiconductor film (polysilicon film), the deposition rate is decreased because a deposition probability of semiconductor atoms (Si) is reduced. In addition, since the thermal decomposition rate of the impurity gas (PH.sub.3) for supplying the impurity (P) is different from the thermal decomposition rate of the raw gas (SiH.sub.4) for forming the semiconductor film in the reaction chamber, the concentration distribution of the impurity gas (PH.sub.3) is locally changed.
A second conventional method of forming a semiconductor film free from a concentration gradient of an impurity gas caused by the difference between thermal decomposition rates will be described below
The second method will be described below by the same apparatus as in the first method.
After a plurality of semiconductor substrates 2 held on the boat 11 are placed in the reaction chamber 1, the reaction chamber 1 is evacuated to have a predetermined pressure and held at a predetermined temperature by the heater 3. Subsequently, only the first valve 9 is opened, and monosilane is supplied to the reaction chamber 1 from the supply port 7 of the first gas supply pipe 5 at a predetermined flow rate. At this time, the second valve 10 is closed. A polysilicon layer having a predetermined thickness thinner than a final thickness is deposited on each of the semiconductor substrates 2. While the first valve 9 is closed, the second valve 10 is opened to supply phosphine to the reaction chamber 1 from the supply port 8 of the second gas supply pipe 6 at a predetermined flow rate. Phosphorus is formed on and added to the polysilicon film deposited on the semiconductor substrate 2. In addition, by the above procedure, monosilane and phosphine are independently and alternately introduced into the reaction chamber 1 to repeat the deposition of polysilicon film and the formation and addition of phosphorus, thereby forming a phosphorus-doped polysilicon film having the predetermined final thickness. Thereafter, the valves are closed, the vacuum of the reaction chamber 1 is broken, and the semiconductor substrates 2 each having the phosphorus-doped polysilicon film are taken from the reaction chamber 1.
However, there are provided the following problems when the phosphorus-doped polysilicon film is formed as described above. According to the method, the uniformity of the films formed on the semiconductor substrates 2 may be improved. However, in order to provide a thick polysilicon film containing a high phosphorus concentration, it must be frequently repeated to introduce monosilane and phosphine into the reaction chamber separately. Therefore, the working time may be increased, and even if films having the same thickness are obtained, the phosphorus concentration cannot be increased more than a predetermined value by the step repetition due to the following reason. Since an amount of adsorption of phosphorus to the polysilicon film is constant, the phosphorus concentration to be added cannot be increased if the film thickness accomplished by one film deposition step is reduced.